Following developments of heavy crude oil and changes of the demand structure for petroleum products around the world, the market shows a rapidly increasing demand for light fuel oil and fast decreasing demand for heavy fuel oil. Heavy oil processing technology has become a crucial R&D subject for oil-refining industry. The heavy oil processing technology mainly comprises decarbonization and hydrogenation.
Decarbonization mainly comprises solvent deasphalting, coking and heavy oil catalytic cracking, etc. Though the facility investment for decarbonization is low, the yield and quality of liquid products are low, and the decarbonization cannot satisfy the current environmental requirements. At the same time, as there is a serious trend that the crude oil is becoming heavier and inferior in quality, the proportion of the residual oil yield against the crude oil is increasing by year, even reaching 70 wt % or above. The decarbonization technology most commonly used for heavy or residual oil is coking, which generally produces a large amount of low value-added coke as side product.
Hydrogenation method, according to the state that catalyst exists in the reactor, is specifically divided to hydrogenation with fixed bed, hydrogenation with moving bed, hydrogenation with suspended bed, and hydrogenation with fluidized bed. In the process comprising hydrogenation, the investment is high because high pressure reaction device is used, but the product quality is good and the liquid yield is high. It may lighten heavy or residual oil at the utmost level. At the moment, the comparatively mature residual oil hydrogenation technology is hydrogenation of residual oil with fixed bed, but this technology is restricted by the nature of the raw material, and has a comparatively strict requirement on some parameters such as metal in raw material and carbon residue etc. Suspended bed and moving bed technologies both have certain advantages in heavy oil processing, but are developed quite slowly in recent years. As in the hydrogenation method with suspended bed a rich amount of heavy metal exists in the tail oil, the processing and utilization of tail oil are very difficult. In the hydrogenation method with moving bed, the crude oil and catalyst will generally pass through the reactor in the same direction or in reverse directions, and heavy oil is processed using the initial activity of catalyst. This method has a fairly good hydrogenation effect, but requires a large amount of catalyst, and the hydrogenation activity of catalyst is not utilized sufficiently.
Currently, fluidized bed hydrogenation technology can realize catalyst's online addition and discharge, be adaptable to a variety of raw material and can guarantee a long-term operation. As such, this technology is developing fast. The fluidized bed reactor is a tri-phase fluid bed, i.e., air, liquid and solid phases. It can treat inferior heavy crude oil with high content of metal and bitumen. It has the characteristics including low pressure drop, homogeneous temperature distribution, constant catalyst activity during the whole operation cycle, and capability of adding fresh catalyst and removing waste catalyst during operation.
The online adding and discharging technology of catalyst is crucial in ensuring product quality, stable operating conditions and long-term operation for the fluidized bed. Currently, on-line catalyst-addition means in the fluidized bed hydrogenation technology generally comprise gas-phase transportation, liquid-phase transportation, or direct addition of solid catalyst from a high-pressure storage tank at the upper part of the reactor to the fluidized bed reactor under gravity. To keep catalyst in the fluidized bed reactor in an excellent fluidizing state, the liquid viscosity, reaction pressure, gas-liquid flow speed and reaction temperature shall be maintained constant in the reactor. Nevertheless, directly adding fresh catalyst into the fluidized bed reactor may easily cause transient fluctuation of the above conditions, resulting in transient instability on fluid state and operating conditions within the reactor. Further, since the initial activity of fresh catalyst is high, adding it directly into the fluidized bed reactor and enabling it to contact and mix with inferior heavy residual oil raw material will result in rapid carbon accumulation of catalyst and fast activity loss, which will influence hydrogenation effect of the reaction flow and increase replacement frequency of catalyst.
CN101418222A, CN1335357A and CN101360808A are some prior arts relating to treatment of inferior residual oil. CN101418222A discloses a combined reaction device of a fluidized bed and a suspended bed. CN1335357A discloses a combined reaction device of an expanded bed and a moving bed. CN101360808A discloses at least two upflow reactors in series. None of these prior arts, however, discloses the on-line treatment of catalyst when the catalyst within the reactor cannot meet the requirement on activity.
U.S. Pat. No. 4,398,852 describes a method for on-line addition of catalyst for a fluidized bed reactor, which comprises the following steps: first adding the catalyst into a high-pressure resistant catalyst-containing container, charging hydrogen into the container to reach the reaction pressure, and opening the valve arranged in the line connecting the catalyst container with the reactor, so that the catalyst enters into the fluidized bed reactor by gravity. In this process, catalyst is directly added into the fluidized bed reactor by gravity, which lead to a rapid carbon accumulation when initially active catalyst contacts with inferior raw material, hence deactivation rate and replacement frequency of catalyst are both enhanced. At the same time, as the temperature pre-heating catalyst and hydrogen is lower than the reaction temperature, the reaction temperature within the fluidized bed will be fluctuated, rendering an unstable operating condition and a low product quality.
U.S. Re 25,770 and U.S. Pat. No. 4,398,852 describe a typical technology of the fluidized bed, in which an inner circulation cup is arranged in the fluidized bed reactor for gas-liquid separation, with which the liquid conversion rate is improved. This technology, however, has the following deficiencies in its practical use: there is a small storage amount of catalyst within the reactor, and the space utilization of the reactor is low; the maintenance of the circulation pump is costly; and once the circulation pump works abnormally or is damaged, catalyst will be sunk and aggregated so that the device has to stop running. Further, when the liquid product in the reactor stays for too long a time under non-catalytic hydrogenation conditions, it will easily be subject to a second thermal cracking reaction to form coke under a high temperature, resulting in an inferior product quality.
CN 02109404.7 and CN101376092A respectively describe a new type of fluidized bed reactor, wherein a tri-phase separator with a guiding aperture is used for effective separation of gas, liquid and solid phases. As compared with typical fluidized bed reactors, it has a simple structure, is easy to operate and has a high utilization rate. Nevertheless, as the height-diameter ratio of the fluidized bed reactor is rather big, generally 1:6 to 1:8, and most of the effective reaction space is an empty tube structure except the tri-phase separator at the top of the reactor, there lacks a positive mass transfer structure, so that the mass transfer effect among gas, liquid and solid is rather poor. Thus, the hydrogenation effect of liquid phase product is insignificant, and the product quality is poor. Further, the fluidized bed reactor is a back-mixing reactor, i.e., part of unreacted raw material is discharged out of the reactor together with reacted flow, hence the conversion rate of raw material is rather low.
There is a fluidized bed in the prior arts which contains two reaction sections or above. Such a fluidized bed reactor has a tri-phase separating component for separating gas, liquid and solid, and it may also realize hydrodemetallization, hydrodesulfurization and hydrodenitrification in sequence, wherein one to two catalysts are used in each reaction section. Said tri-phase separating component consists of a flow-guiding element and a flow-blocking element, wherein the flow-guiding element is a taper or a cone that opens at both ends with one opening smaller than the other, and an upper flow-guiding element and a lower flow-guiding element are used, where the upper end of the lower flow-guiding element is coaxially matched with the lower end of the upper flow-guiding element. Such a reactor is actually a large reactor formed by a combination of two reactors, with the lines connecting the reactors and other devices such as separators and sinking tanks removed. Although with the advantage that heat energy can be reasonably utilized, it still has the following disadvantages: the reactor is huge, causing difficulties in transportation, installation, operation and maintenance; with the increase of reaction sections in one reactor, the number of tri-phase separators also increases, hence the structure of the whole reactor is complicated; also, the more tri-phase separators, the more space they will occupy in the reactor, and the smaller effective reaction space for gas, liquid and solid there will be left, which means the resources will be wasted since the fluidized bed reactor operated under high temperature and high pressure is very costly; further, for the operation under multiple sections connected in series, it requires a stable operating condition and a very good design of gas-liquid distribution plate between sections, and when there are transient fluctuations, the separating effect of the tri-phase separator will be influenced, such that the solid entrained with the gas and liquid will block the distribution plate and influence the normal stable operation of the device.